1. Field of the Invention
The present invention relates to an ITU/TDD/VOICE modem and, more particularly, an ITU/TDD/VOICE modem that may operate in a stand alone mode, which includes an answering machine mode and a prestored outgoing message mode.
2. State of the Art
The telephone system or "public switched telephone network" (PSTN) was first developed in the late 1800's to allow for person to person communications by voice over long distances. The invention of computers made it useful to exchange digital data over the PSTN. In the 1950's, "modems" were developed for that purpose. A modem accepts digital data, modulates it into a form suitable for transmission over the PSTN such that a corresponding receiving modem can demodulate the signal back into the original digital data.
Modems provide a means for the deaf or speech impaired to communicate over the telephone system, since typed characters can be sent over the PSTN via modem and read by another modem user. However, standard modems have evolved with poor provision for communications by the deaf or speech impaired and are largely unsuitable for this purpose. Standard modems were developed primarily for computer to computer communications. For example, most standard modems have a "carrier" which is a constant audible signal. If this signal is lost because the call is placed on hold, a standard modem will generally hang up and cannot reestablish the connection with a different modem. Modems are very inflexible in their protocol requirements and will not operate properly or will abort the call if the protocol settings are wrong.
In the late 1960's, three deaf engineers developed a new type of very simple modem called a TDD or "telecommunication device for the deaf", sometimes called a TTY. A TDD is typically a stand alone modem system with an integral keyboard and display that allows typed characters to be exchanged with another TDD over the PSTN. Because of its simple protocol requirements, the TDD is robust and easy to use. Consequently, the TDD is, in many respects, more suitable than are standard modems for communications by the deaf.
The International Telecommunications Union ("ITU") (formerly known as the CCITT) has been active in setting various modulation standards for modems. Examples of ITU standards are V.21, V.22, V.22 Bis, V.32, V.32 Bis, and V.34. In the United States, prior to the adoption of ITU standards, various "Bell standards" were established. Examples of Bell standards are Bell 103 and Bell 212. As used herein, an "ITU modem" is a modem that may operate using an ITU modulation/demodulation protocol (or standard). Typically, ITU modems may also operate in at least one Bell modulation/demodulation protocol. A "Bell modem" may operate in a Bell modulation/demodulation protocol. Most modems currently being sold in the United States are ITU modems that are capable of transmitting and receiving in various ITU modulation/demodulation protocols as well as Bell modulation/demodulation protocols. The term "ASCII modem" is sometimes used to refer to an ITU or Bell modem.
As used herein, a "standard modem" is a modem other than a TDD that follows an established protocol such as an ITU or Bell modulation/demodulation protocol and transmits and/or receives data over a communication medium. The communication medium may be a telephone system (such as the PSTN), private branch exchange (PBX), or other media such as microwave link, coax, or fiber optic cable. ITU modems and Bell modems are examples of standard modems.
Differences between standard modems and TDDs include: (1) TDDs encode text characters with the generally obsolete five bit Baudot code, while most modems and desktop computers in the United States today use an ASCII encoding, also known as the ANSI X3.4-1977 or ASCII-77 encoding, (2) TDDs do not provide for parity bits, (3) TDD's in the United States transmit/receive only at 45.45 bits per second, and (4) TDDs use frequency shift keying modulation/demodulation at frequencies of 1400 and 1800 Hz. Because TDDs do not use ITU modulation and protocol schemes, TDDs and ITU modems are incompatible and cannot communicate with each other.
Dual purpose modems have been developed that can communicate with both TDDs and ITU modems. Some of these dual purpose modems have the capability to determine whether incoming and outgoings signals are in TDD format or standard modem format.